Optimizing complexity in the kinetic modelling of integrated flue gas purification for pressurized oxy-combustion

Abstract

A 5-step mechanism and a corresponding kinetic model are presented for integrated flue gas purification, where SOx and NOx are removed during pressurized oxy-coal combustion. Three coupled subsystems (‘blocks”) of gas–liquid transformations are considered: 1) oxidation of NO to NO2 and dissolution of NO2 in water; 2) dissolution of SO2 in the water; and 3) interactions between S- and N- containing substances in the liquid phase. The model has been formulated in terms of only species that can be experimentally measured (model of optimal complexity). Subsystem I is limiting regarding NO removal. Subsystem II is very fast in comparison with the other subsystems and can be treated separately. Analytical expressions for all independent subsystems have been found and compared with computational results for the whole system. The temporal domain of validity of all these subsystems is presented. Finally, the validity of the NO2 quasi-steady-state assumption is verified, and it is reported that variables such as NO, NO2 and HNO3 can be considered decoupled from the whole system since they appear in only Subsystem I, while HSO3− and HNO2 are coupled. The presented kinetic model was verified experimentally by determining the 1:1 ratio of HNO2 and HNO3, in gas–liquid NO-O2 interaction. Additionally, new experimental facts on kinetic orders and stoichiometry between HNO2 and HSO3− have been used to optimize the model. It is concluded that all NOx- SOx interactions can be essentially grasped by a sequence of only 4 irreversible reactions considering, in addition, the 5th reaction, i.e. the fast ‘SO2-water’ equilibrium.